In the field of electrophotography, attempts have recently been made to utilize organic materials for electrophotographic photoreceptors. The organic photoreceptor is composed of a combination of a material capable of generating a charge (hereinafter referred to as "charge generating material") and a material capable of transporting a charge (hereinafter referred to as "charge transport material"), in which a charge generating layer containing a charge generating material and a charge transport layer containing a charge transport material are laminated on a support to form a photosensitive layer, or a photosensitive layer having a charge generating material dispersed in a charge transport material is formed on a support, as described, for example, in U.S. Pat. Nos. 4,559,286, 4,555,463, and 4,552,822.
The charge to be transported includes positive holes and electrons. Most of the conventionally proposed charge transport materials are capable of transporting positive holes only, as exemplified by pyrazolines, hydrazones, and the like, while there is only a mixture of polyvinylcarbazole (PVK) and trinitrofluorenone (TNF) known as a practically employable charge transport material capable of transporting electons. Since the conventional photoreceptors using positive hole-transport materials are composed of a support, a charge generating layer, and a charge transport layer in this order, they should be charged negatively. Such negatively working photoreceptors are unavoidably susceptible to chemical denaturation due to negatively charged ozone and, therefore, they have considerably poor printing durability as compared with inorganic photoreceptors containing amorphous selenium, amorphous silicon or other inorganic materials. Besides, they fail to exhibit capability of being charged both positively and negatively, i.e., bipolarity. From these considerations, it has been demanded to develop an electrophotographic photoreceptor capable of transporting electrons, and more desirably a bipolar photoreceptor.
Tetracyanoanthraquinodimethane (hereinafter abbreviated as "TCNAQ") and its derivatives are known as organic compounds chargeable to both polarities as described in Japanese Patent Application (OPI) Nos. 49259/82, 10554/83, and 55450/83 (the term "OPI" as used herein means "unexamined published Japanese patent application").
The TCNAQ derivatives have a basic skeleton represented by formula: ##STR2## to which various substitutents are bonded.
These compounds are useful as organic electronic materials, such as organic conductors, thermistor materials, etc., but have a disadvantage of low electrical conductivity.
The TCNAQ derivatives have been conventionally prepared through a series of steps as shown below as disclosed in Japanese Patent Application (OPI) No. 10554/83: ##STR3##
According to the above-illustrated process, a number of steps are involved, the reaction time required is long, the operation is complicated, and the yield is low. In addition, since the resulting product has a low purity, it produces many trap sites, when used as a charge transport material of electrophotographic photoreceptors, resulting in the failure of attaining a low residual potential and suppressing changes with time.